A conventional gear tooth sensor [1] consists of an IC (integrated circuit) 11 that includes Hall effect sensors 12 together with a single hard magnet 13, as shown in FIG. 1. The IC supports two Hall sensors, which sense the magnetic profile of the ferrous target simultaneously, but at different points, thereby generating a differential internal analog voltage that is further processed for precise switching of the digital output signal. To achieve a high differential signal output, the two Hall probes (or sensors) are spaced at a certain distance so that one Hall sensor faces field concentrating tooth 14 and the other Hall sensor faces gap 15 in the toothed wheel. A permanent magnet mounted with one pole on the rear side of the IC produces a constant magnetic bias field.
If one Hall sensor momentarily faces a tooth while the other faces a gap between teeth, the gear tooth acts as a flux concentrator. It increases the flux density through the Hall probe and a differential signal is produced. As the gear wheel turns, the differential signal changes its polarity at the same rate of change as from the tooth to the gap. An integrated highpass filter regulates the differential signal to zero by means of a time constant that can be set with an external capacitor. In this way only those differences that changed at a minimum rate are evaluated. The output signal is not defined when in the steady state.
A GMR based gear tooth sensor has also been proposed in which the sensing structure is similar to traditional Hall IC based gear tooth sensor except that the two Hall probes are replaced by two GMR sensors [2], as shown in FIGS. 2a-d. Due to the high sensitivity of GMR devices, the GMR based gear tooth sensors provide a very large output signal which is stable over the rated temperature and voltage range. As a result, GMR based gear tooth sensors feature excellent air-gap performance and an extremely stable operating envelops as well as robust reliability characteristics.
As shown in FIGS. 2a-d, the magnetic field generated by the bias magnet is influenced by the moving gear tooth, the GMR sensors serving to detect the variation of the magnetic field component within the GMR film plane. The signal output is then generated from differential signals from two GMR sensors or a GMR bridge. Since the permanent magnet is mounted with either pole on the rear side of the GMR sensors (as in the Hall IC based gear tooth sensor) the magnetic field is essentially perpendicular to the GMR films. So any variation in the mechanical placement of the permanent magnet or any tilting of the GMR sensors during assembly could result in a large offset field in the GMR film plane. As a result, degradation may occur. Also, in this prior art design, the two GMR sensors have to be placed a certain distance apart (˜0.5 mm to 5 mm), depending on the particular gear tooth dimensions, so they cannot be initially formed on the same wafer. Instead, the individual GMR sensors must first be formed separately and then assembled along with other components of the IC. This is expensive and prone to causing additional variations from one installation to the next.